The present invention relates to wireless communications systems and methods, and more particularly, to apparatus and methods for increasing range in wireless communications systems.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 (superseded by IS-136) and the European standard GSM (Global System for Mobile Communications) have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular communication system 20. The cellular system 20 may include one or more terminals 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of terminals.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between terminals 22 and the MISO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two terminals 22 or between a terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
Those skilled in the art will appreciate that xe2x80x9ccellsxe2x80x9d may have configurations other than the omnidirectional cells 24 illustrated in FIG. 1. For example, the coverage areas conceptually illustrated as a hexagonally-shaped area served by a base station 26 may actually be subdivided into three sectors using separate directional antennas mounted at the base station 26, with the sector antenna having patterns extending in three different directions. Each of these sectors may itself be considered a xe2x80x9ccellxe2x80x9d As will be appreciated by those skilled in the art, other cell configurations are also possible, including, for example, overlaid cells, microcells, picocells and the like.
Cell size in time-division multiplexed communications systems is typically limited by the effect of propagation delays on synchronizing the arrival of transmissions from variously located terminals to the slotted frame structures used by base station transceivers. In order to synchronize transmissions from terminals located in a cell, the base station terminal typically transmits a respective timing advance value (TA) to a respective terminal. The terminal advances its transmissions to the base station according to the timing advance value to compensate for the propagation delay between the terminal and the base station. Typically, the timing advance values instruct the terminals to advance their uplink transmissions such that the transmissions from all the terminals served by a base station arrive at the base station in synchronism with a common receive frame structure.
When a terminal attempts to access a system, however, such propagation delay information typically is unavailable. Accordingly, conventional time-division-multiplexed systems commonly utilize a random access channel (RACH) to receive an access request burst from such an unsynchronized terminal and use propagation delay gained from the received RACH burst to determine an appropriate timing advance for the terminal. Upon powering up or handoff to a new base station, an unsynchronized terminal searches for and receives a control channel from the base station that provides an initial timing reference. To initiate use of the base station, the terminal then transmits a RACH burst at a predetermined time in relation to the control channel timing reference. Upon receipt of the RACH burst, the base station can determine round-trip time delay based on the delay between the transmission of the control channel timing reference and the receipt of the RACH burst. The base station uses this round-trip time delay to determine an appropriate timing advance for the terminal.
The RACH typically is a slotted channel that is designed to tolerate significant variation in RACH burst timing. Each RACH slot typically includes a significant amount of xe2x80x9cguard timexe2x80x9d so that RACH bursts in adjacent slots are less likely to overlap. The amount of guard time provided typically limits maximum cell size, as the amount of guard time determines the maximum delay variation in RACH bursts that can be received by a base station.
For example, in systems conforming to the GSM recommendations, cell size is typically limited by: (1) the number of guard bits (68.25) provided in slots assigned to a RACH logical channel for random access bursts; (2) the number of bits (6) allotted to the timing advance message field in slow associated control channel (SACCH); and (3) synthesizer switching time required between receipt and transmit bursts at terminals operating in half-duplex mode. RACH bursts are used by terminals to achieve access, e.g., at handoff or initial access, and typically have relatively long guard periods (68.25 bits or 252 xcexcsecs). Using an 8.25 bit guard time, the remaining 60.0 bit period (221.5 xcexcsec) of a slot is available for roundtrip time estimation. The roundtrip delay between a terminals and a base station should be within 221.5 xcexcsec; otherwise, a RACH bust may overlap and/or collide with the next time slot. The maximum of 221.5 xcexcsec roundtrip delay thus generally provides for a maximum cell radius of 33.2 km. For cells larger than 33.2 km distance, a RACH burst may collide with the next slot burst, and thus may not allow the base station to estimate the correct roundtrip delay and decode the RACH burst.
Once a connection has been established between a terminal and a base station in a GSM system, the base station continues to measure the time offset between its own burst schedule and bursts received from the terminal. Based on these measurements, the base station periodically provides the terminal with timing advance information in the form of a 6-bit timing advance value (TA) transmitted on the slow associated control channel (SACCH) at a rate of twice per second. The base station estimates round-trip delay on the random-access channel (RACH) on the common control channel (CCCH), and uses this estimated round-trip delay to determine the appropriate timing advance value to send to the terminal. Typically the timing advance value sent by the base station corresponds to the absolute delay between the base station and the terminal in terms of the number of bit periods, such that the 6-bit timing advance value provides a range of from 0 bit periods to 63 bit periods of advance, with a resolution of 1 bit period.
Referring to FIGS. 2A and 2B, the uplink frame in GSM is typically delayed by 3 slot periods with respect to the downlink frame (GSM slots have a length of 577 xcexcsec, and include 156.25 bit periods). A terminal is typically assigned a slot pair, i.e., a single slot in each of the frames of the downlink and uplink carrier frequencies. Assuming an upper bound on propagation delay, the time separation between the downlink and uplink slots allows the terminal to use a simple half-duplex mode of radio operation wherein the terminal switches between receiving and transmitting on the two different carrier frequencies. A terminal assigned a maximum 63 bits of timing advance and operating in half-duplex mode typically has about 1xc2xd slots for frequency synthesizer switching from receive to transmit.
If there is no delay, the base stations sends a timing advance value TA=0, and the terminal transmits to the base station using a transmit frame structure that lags its receive frame structure by 468.75 bits (3 slot periods). At the maximum delay that can be compensated for by the 6-bit timing advance value TA in the SACCH field, the base station commands a timing advance TA=63, and the terminal transmits using a transmit frame structure that lags its receive frame structure by 405.75 bits. In such a system, with the maximum of 63 bit periods of timing advance, the roundtrip delay between the terminal and the base station is limited to 233 xcexcsecs, giving a maximum distance of 34.9 km.
As described above, the extended guard time need for RACH bursts, along with the limits on the timing advance field in SACCH message and terminal synthesizer switching time limitations, typically limit cell size in GSM to approximately 35 km. Similar limitations typically are also present in other time-division multiplexed systems. Although conventional cell sizes may be sufficient for many if not most applications, there are many applications in which larger cell sizes may be advantageous. For example, in rural areas having low-density user populations, larger cells may reduce the number of base stations needed to cover a region, and thus lower capital and operating costs. Similarly, large cells may be advantageously used for long, straight highways and similar applications in which users in a large area are constrained to relatively small portion that extends significantly in only one direction. Larger cells may also be useful in locations where physical geography limits the number of acceptable base station sites, such as in coastal areas.
One proposed approach for increasing cell size involves restricting assignment of users to every other slot of the receive frame structure, such that the unused slots provide additional guard time to compensate for increased propagation delay. Unfortunately, such an approach can significantly reduce system capacity. This inefficiency may be particularly significant when only a relatively small number of users are located at a significant distance from a base station.
Another approach to increasing cell size in GSM systems is to transmit relative timing advance values to terminals instead of absolute timing advance values. According to such an approach, the timing advance value sent by a base station is interpreted as an increment or decrement by which a terminal is to increase or decrease its timing advance. Applied to a GSM system, for example, the 6 bits provided for the TA value in the SACCH message is large enough to support much larger cell sizes if used to communicate relative timing advance values. However, for terminals utilizing a half-duplex mode of operation, this scheme generally is limited by minimum synthesizer switching time, i.e., the transmit frames of the terminal cannot be advanced to the point that the terminal has insufficient time to switch between the receive and transmit frequencies. Faster synthesizer switching time may be achieved, but typically with a significant increase in the cost of terminals. In addition, using a relative timing advance does not address limitations to the guard times provided in the RACH, and may require modification of the existing air interface standard and terminal hardware.
In light of the foregoing, it is an object of the present invention to provide wireless communications apparatus and methods that can provide increased range.
It is another object of the present invention to provide wireless communications apparatus and methods that can provide increased range using existing terminals and air interfaces.
These and other objects, features and advantages are provided according to the present invention by wireless communications apparatus and methods in which respective groups of terminals, e.g., respective groups of terminals at respective different ranges with respect to a base station, are instructed to time their transmissions to arrive at the base station in synchronism with respective time-offset frame structures. Transmissions from respective groups may be transmitted on respective uplink carrier frequencies, synchronized to respective time-offset series of frames. Transmissions from respective groups may also be transmitted on a common uplink carrier frequency, and received in synchronism with respective frame series that are time-multiplexed on the common uplink carrier frequency. According to another aspect of the invention, overlapping ranges may be defined, such that hysteresis in switching between frame structures may be provided when a terminal moves between the ranges.
By providing staggered frame structures, the present invention allows the use of larger cell sizes. Transmissions from distant terminals may be received at a base station in synchronism with frame structures that are delayed with respect to the frame structures used to receive transmissions from less distant terminals, obviating the need to use large timing advances for the more distant terminals. This staggered approach obviates the need for faster synthesizer switching times, and allows for the use of existing terminals and air interfaces. As the transmissions from the groups are separated in either time or frequency, collisions between signals may be avoided.
In particular, according to an embodiment of the present invention, a first group of terminals is instructed to time their transmissions to arrive at a base station in synchronism with a first series of frames. A second group of terminals is instructed to time their transmissions to arrive at the base station in synchronism with a second series of frames that is time-offset with respect to the first series of frames. Preferably, he first group of terminals is located at a first range with respect to the base station, and the second group of terminals is located at a second range with respect to the base station. Transmissions from the first and second groups of terminals are received at the base station in synchronism with the respective first and second series of frames. The transmissions from respective first and second groups may be received on respective separate carrier frequencies, or the first and second series of frames may be multiplexed on a common carrier frequency.
In one embodiment of the presentation invention, first timing information is transmitted from the base station, instructing a terminal of the first group to time its transmissions to arrive at the base station in synchronism with the first series of frames. Second timing information is transmitted from the base station, instructing a terminal of the second group to time its transmissions to arrive at the base station in synchronism with the second series of frames. The transmitted timing information may include a timing advance value that is determined based on a propagation delay between a terminal and the base station. The propagation delay may be determined from timing of a random access channel (RACH) burst.
According to another aspect of the present invention, first timing information is transmitted by a base station to a terminal when the terminal is in a first range, the first timing information instructing the terminal to time its transmissions to arrive at the base station synchronized to a first series of frames. The base station transmits second timing information to the terminal when the terminal is in a second range, the second timing information instructing the terminal to time its transmissions to arrive at the base station synchronized with a second series of frames that is time-offset with respect to the first series of frames. The ranges may overlap, and hysteresis may be provided in instructing the terminal to synchronize its transmissions to one of the first series of frames or the second series of frames when the terminal moves between the first and second ranges.
According to another aspect of the present invention, an apparatus in a time-division multiplexed wireless communications system includes means for instructing a first group of terminals to time their transmissions to arrive at a base station in synchronism with a first series of frames, and means for instructing a second group of terminals to time their transmissions to arrive at the base station in synchronism with a second series of frames that is time-offset with respect to the first series of frames. The apparatus further includes means for receiving transmissions from the first and second groups of terminals at the base station in synchronism with the respective first and second series of frames.
In another embodiment of the present invention, a base station for a time-division multiplexed wireless communications system includes means for transmitting first timing information to a terminal when the terminal is in a first range with respect to a base station, the first timing information instructing the terminal to time its transmissions to arrive at the base station synchronized to a first series of frames, and means for transmitting second timing information to the terminal when the terminal is in a second range, the second timing information instructing the terminal to time its transmissions to arrive at the base station synchronized with a second series of frames that is time-offset with respect to the first series of frames. The base station may further include means for receiving a transmission from the terminal in synchronism with the first series of frames on a first carrier frequency, and means for receiving a transmission from the terminal in synchronism with the second series of frames on a second carrier frequency. Alternatively, the base station may include means for receiving a transmission from the terminal in synchronism with one of the first and second series of frames on a common carrier frequency.
In yet another embodiment according to the present invention, a wireless communications apparatus includes a base station operative to transmit first timing information that instructs a first group of terminals to time their transmissions to arrive at a base station in synchronism with a first series of frames, and to transmit second timing information that instructs a second group of terminals to time their transmissions to arrive at the base station in synchronism with a second series of frames that is time-offset with respect to the first series of frames. The base station is further operative to receive transmissions from the first and second groups of terminals in synchronism with the respective first and second series of frames.
The bases station also may be operative to transmit first timing information to a terminal when the terminal is in a first range with respect to a base station, the first timing information instructing the terminal to time its transmissions to arrive at the base station synchronized to a first series of frames, and to transmit second timing information to the terminal when the terminal is in a second range, the second timing information instructing the terminal to time its transmissions to arrive at the base station synchronized with a second series of frames that is time-offset with respect to the first series of frames. The first and second ranges may overlap and the base station may provide hysteresis in instructing the terminal to synchronize its transmissions to one of the first series of frames or the second series of frames when the terminal moves between the first and second ranges.